function keyboardMap(digits){
    let map =['','abc','def','ghi','jkl','mno','pqrs','tuv','wxyz']
    let result = []
    for (let i = 0;i<digits.length;i++){
        result = _compose(result,map(digits[i]))
    }
    function _compose(arr1,arr2){
        var r = [];
        if(arr1.length ==  0) return arr2;
        if(arr2.length ===0 )return arr1;
        for(let i = 0;i<arr1.length;i++){
            for(let j = 0 ;j<arr2.length;j++){
                r.push(arr1[i]+arr2[j])
            }
        }
        return r
    }
}